When two intermediate chemical equations are combined, the same substance that appears in the same phase can be canceled out, provided that

When adding chemical equations, a substance cancels if it appears on opposite sides of the combined equations: as a product in one equation and a reactant in the other. It must be the same substance in the same phase so the terms match exactly. Therefore, the correct choice is “it is a reactant in one intermediate reaction and a product in the other reaction.”

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